Десятилетие sounds rather formal and official. It is rarely used in common speech, so it doesn't suit any of your examples.
(1) Десяток is preferred, of course
Wait, they've been married for almost 2 decades?! --> Погоди, так они женаты уже почти два десятка (or simply двадцать) лет?!
(2) 50s, 60s, 70s, etc. are naturally translated into пятидесятые, шестидесятые, семидесятые etc. Note that word года is silently assumed but rarely spoken here (i.e. пятидесятые года)
Are you a fan of 80s music? --> Тебе нравится музыка восьмидесятых?
(3) If Russian speaker means "about X years", he simply changes word order. So "ten years" = "десять лет", but "a decade" = "лет десять". Also note that negative sentence is preferred here.
A decade has passed since I last saw him --> Я его не видел уж лет десять
(4) Again десяток is normally used if talking about human's age
Michael is in his 50s --> Михаил разменял шестой десяток